Process Variability and Electrostatic Analysis of Molecular QCA

Abstract

Molecular quantum-dot cellular automata (mQCA) is an emerging paradigm for nanoscale computation. Its revolutionary features are the expected operating frequencies (THz), the high device densities, the noncryogenic working temperature, and, above all, the limited power densities. The main drawback of this technology is a consequence of one of its very main advantages, that is, the extremely small size of a single molecule. Device prototyping and the fabrication of a simple circuit are limited by lack of control in the technological process [Pulimeno et al. 2013a]. Moreover, high defectivity might strongly impact the correct behavior of mQCA devices. Another challenging point is the lack of a solid method for analyzing and simulating mQCA behavior and performance, either in ideal or defective conditions. Our contribution in this article is threefold: (i) We identify a methodology based on both ab-initio simulations and post-processing of data for analyzing an mQCA system adopting an electronic point of view (we baptized this method as “MoSQuiTo”); (ii) we assess the performance of an mQCA device (in this case, a bis- ferrocene molecule) working in nonideal conditions, using as a reference the information on fabrication-critical issues and on the possible defects that we are obtaining while conducting our own ongoing experiments on mQCA: (iii) we determine and assess the electrostatic energy stored in a bis-ferrocene molecule both in an oxidized and reduced form. Results presented here consist of quantitative information for an mQCA device working in manifold driving conditions and subjected to defects. This information is given in terms of: (a) output voltage; (b) safe operating area (SOA); (c) electrostatic energy; and (d) relation between SOA and energy, that is, possible energy reduction subject to reliability and functionality constraints. The whole analysis is a first fundamental step toward the study of a complex mQCA circuit. It gives important suggestions on possible improvements of the technological processes. Moreover, it starts an interesting assessment on the energy of an mQCA, one of the most promising features of this technology.